The best replacement for 91 is 93 octane.

The best alternatives to 91 are rated as following:
★★★★★  93 octane
★★★  87 octane
★★★  95 octane
★★  70 isopropyl alcohol
★★  95
★★  99 isopropyl alcohol
★  94 octane
★  e85
★  part 135
★  98 octane
★  87
★  89 octane
★  93 gas
★  135
★  fal
★  m1a
★  93
★  100 octane
★  82
★  96 octane
  87 octane gas
  70 alcohol
  95 unleaded
  ar 10
  fn fal
  cetme
  hk 91
  sa58
  87 gas
  201
  os 91
  dsa fal
  201 2
  95 gasoline

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Speaking about 91, one probably shouldn't forget about block variable not set.

Other search terms related to 91 and their ratings are:
★★★★★ block variable not set
★★ block not set
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91
93 octane
87 octane